Site Contents
- Introduction
- Audio Data Format
- Interface Applications
- Hardware Specifications
- Data Transmission
- Digital-Analog Converter
- Resetting & Auto-Switching
- History & Characteristics
- Data & Cabling Details
- Coding Format
- Conversion Circuits
- Internal Connection
- External Connection
- Optical Output Circuit
- Transmission Line
Sponsors
SPDIF: Transmission Line
Components
There are seven components in SPDIF transmission line that excludes the driver, receiver and termination. One is the transport circuit board traces connecting to the driver chip. Then there is the wiring to the output connector, the output connector BNC or RCA jack and plug and the digital cable. Then at the DAC input, there is input connector BNC or RCA jack/plug. Also there is the circuit board wiring and DAC circuit board traces that is connected to the receiver chip.
Impedance
Reflection on the transmission line may be caused by any of the above components if not of 75 ohm impedance character. It is therefore not adequate to replace RCA connectors with BNC if the impedance issue is not addressed in respect of wiring and circuit board traces. Transport or DAC circuit board hardly has impedance control. However sometimes as an exception, circuit board to connector wiring is made in a way to control impedance.
Reflection
Characteristic impedance cannot be measured by volt-ohmmetre. It depends on the inductance and capacitance per unit length of the transmission line. Transmission of digital signals over long distance can be reflection-free only if there is proper matching and termination in transmission line. The rise-time in the voltage swing causes reflection on the transmission line. A low rise-time of say 50 nanoseconds prevents reflection. Less than 1 nanosecond rise-time causes reflection at each boundary. It takes a certain period of time for the transition to propagate to the other end. This time is about 2 nanoseconds/foot, that is, slower than speed of light but can be longer depending on digital cable dielectrics. A reflection can occur when the transmission reaches the end of the DAC. This reflection is propagated back to the transport driver. After reaching the driver, it can get reflected back to DAC. These reflections finally decay after 3-4 instances when best low-loss digital cables are used.